CA1321046C - Method for dewatering paper - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method of enhancing the dewatering of paper during a papermaking process which includes the steps of adding a low molecular weight cationic coagulant and then colloidal silica and a high molecular weight flocculant.

Description

2 13210~ 66530-452 Field ~ o~_~D---e1~
The fielcl of the invention is papermaking. More particularly, the inventlon relates to a process for improving the dewatering of paper.
Backqround of the Invention Paper is made by applying processed paper pulp to a Fourdrinier machine. In the papermaking process, it is necessary to drain water from the paperstock. The use of collodial silica together with cationic starch has proved -~
beneficial in providing drainage.
It would be advantageous to provide a drainage method with improved results.
Summarv of the Invention Accordtng to the present invention there is provided a method for dewatering paper furnish in a papermaking process, which method comprises the steps of adding to the paper furnish from 0.1 to 25 pounds per ton on a dry basis, hased on furnish, of a low molecular weight cationic organic polymex having a molecular weight of at least 1000 up to about 500,000; and then adding collodial silica with an average particle size within the range of from 1 to 100 nm; and 0.1 to 5 pounds per ton on a dry ba~is, based on furnish, of a high mo]ecular weight charged acrylamide copolymer having a molecular weight of at least 500,000, the collodial silica and the high molecular weigh~
charged acrylamide copolymer being added in either order.
The low molecular weight (LMW) cationic polymers will be positively charged polymers having a molecular weight of at lea~t 1000 and should have a molecular weight less than 500,000. Polymers having molecular weights in ~he range 2,000 to 200,000 are preferred. Preferred polymers include epichlorohydrin/dimethylamine (epi/DMA) and ethylene dichloride/ammonia copolymers (EDC/NH3~, ~, . , 132104~
2a 66530-452 diallyldimethylammonium chloride (polyDADMAC) copolymers and acrylamido N,N-dlmethyl piperazine quaternary/acrylamide copolymers.

1 3 2 1 ~ 4 6 The high molecular weight (HMW) charged polymers are preferably acrylamide polymers which can include either cationic monomers or anionic monomers. Generally they will have a Mw of at least 500,000. Polymers having a molecular weight greater than 1,000,000 are most preferred.
The low molecular weight cationic polymer preferably will be fed on a dry basis at 0.1 to 25lb. per ton of paper furnish. More preferably the low molecular weight polymer will be fed at 0.2 to 10 lb. per ton of paper furnish.
The high molecular weight charged acrylamide copolymer should be fed at 0.1 to 5 lb. per ton furnish on a dry basis, more preferably at 0.2 to 3 lb. per ton furnish.
Description of the Preferred Embodiments A low molecular weight cationic polymer is added to paper feedstock and tends to neutralize the charge on the paper feedstock to facilitate coagulation thereof. Subsequent to this addition of low molecular weight polymer, a high molecular weight polyacrylamide and colloidal silica are added to the paper feedstock. The process will work regardless of the order of addition of the silica and the high molecular weight polymer with respect to each other. However, the order may be important for optimization of performance and that optimal order can vary with the mill system being treated.
Anionic High Molecular Weight Elocculants The high molecular weight anionic polymers are pre-ferably water-soluble vinylic polymers containing monomers from the group acrylamide, acrylic acid, AMPS 2-acrylamido-2-methyl-propane sulfonate and mixtures thereof, and may also be either hydrolyzed acrylamide polymers or copolymers of acrylamide or its homologues, such as methacrylamide, with acrylic acid or its - ~ - 1 3 2 1 0 4 6 66530-452 homologues, such as methacrylic acid, or with monomers, such as maleic acid, itaconic acid or monorners such as vinyl sulfonic acid, AMPS, and other sulEonate containing monomers. The anionic polymers may be homopolymers or copolymers, including terpolymers.
The anionic polymers may also be sulfonate or phosphonate con-taining polymers which have been synthesized by modifying acryl-amide polymers in such a way as to obtain sulEonate or phosphonate substitution, or mixtures thereof.
The most preferred high molecular weight copolymers are acrylic acid/acrylamide copolymers and sulfonate containing polymers such as 2-acrylamido-2-methylpropane sulfonate/acryl-amide; acrylamido methane sulfonate/acrylamide; 2-acrylamido ethane sulfonate/acrylamide and 2-hydroxy-3-acrylamide propane sulfonate/acrylamide. Commonly accepted counter ions may be used for the salts such as sodium ion, potassium ion, etc. The acid or the salt form may be used but it is preferred to use the salt form of the charged polymers.
The anionic polymers may be used in solid, powder form, aqueous, or may be used as water-in-oil emulsions where the polymer is dissolved in the dispersed water phase of these emulsions.
It is preferred that the anionic polymers have a mole-cular weight of at least 500,000. The most preferred molecular weight is at least 1,000,000 with best results observed when the molecular weight is between 5-30 million. The anionic monomer should constitute at least 2 mole percent of the copolymer and more preferably the anionic monomer will constitute at least 20 mole percent of the over-all anionic high molecular weight polymers, i.e. the anionic copolymer should have a degree of anionic substitution of 2 mole percent, preferably 20 mole percent. By degree of anionic substitution, we mean that the 132~0~6 66530-452 polymers contain randomly repeating monomer units containing chemical functionality which when dissolved in water become anionically charged, such as carboxylate groups, sulfonate groups, phosphonate groups, and the like. As an example a copolymer of acrylamide (AcAm) and acrylic Acid (AA) wherein the AcAm:AA
monomer mole ratio is 90:10, would have a degree of anionic sub-stitution of 10 mole percent. Similarly copolymers of AcAm:AA
with monomer mole ratios of 50:50 would have a degree of anionic substitution of 50 mole percent.
Cationic High Molecular Weight Polymer Flocculants The cationic polymers used are preferably high molecular weight water soluble polymers having a weight average molecular weigh'c of at least 500,000, preferably a weight average molecular weight of at least 1,000,000 and most preferably having a weight average molecular weight ranging from about 5,000,000 to 25,000,000.
Examples of suitable high molecular weight cationic polymers include diallydimethyl ammonium chloride/acrylamide copolymers; l-acryloyl-4-methyl-piperazine methyl sulfate quat/(AMPIQ) acrylamide copolymers; dimethylaminoethylacrylate quaternary/acrylamide copolymers (DMAEA); dimethyl aminoethyl methacrylate quaternary acrylamide copolymer (DMAEM), meth-acrylamido propyl trimethylammonium chloride homopolymer (MAPTAC) and its acrylamide copolymer.
It is generally preferred that the cationic polymer be an acrylamide polymer with a cationic comonomer. The cationic comonomer should represent at least 2 mole percent of the overall polymer, more preferably, the cationic comonomer will represent at least 20 mole present of the polymer.

The Ols~ersed Sllica Preferably, thc cationic or anionic polymers are used in combination with a dispersed silica having an average partlcle size ranging between about 1-100 nanometers (nm), preferably having a Particle size ranging between 2-25nm, and most preferably having a particle size ranging between about 2-15nm.
This dispersed silica, may be in the form of colloidal, silicic acid, silica sals, fumed silica,agglomerated silicic acid, silica gels, and precipitated silicas, as long as the particle size or ultimate particle size is within the ranges mentioned above. The dispersed silica is normally present at a weight ratio of cationic coagulant (i.e. LMW cationic polymer) to silica of from about lûO:l to about 1:1, and is preferably present at a ratio of from 10:1 to about 1:1.
This combined admixture is used within a dry weight ratio of from about 20:1 to about 1:10 of high Mw polymer to silica, Preferably between about 10:1 to about 1:5, and ~ost preferably between about 8:1 to about 1:1.
The following examples demonstrate the method of this invention.

Example 1 500 mls. paPer stock mixed with the additlves in the followlng order of additlon:
1. low molecular weight cationic polymer;
2. hlgh molecular weight polymer }. colloidal silica These samples were mixed after each addition of chemicals in a 500 ml. graduated cylinder, then the samples received 3 seconds ~ixing at 1000rPm. The samples were then drained throu9h a laboratory dralnage tester; the first 5 seconds of filtrate being collected for testinq. The results are provided in Table I.

1~210~6 Table I
(lb/ton)*
HMW Polymer Cationic LMW Polymer Colloidal Orainaae Product Drytlb/ton) Starch Product Drytlb/ton) Silica 270 mLs/5sec 110 0.5 200 1.3 175 110 0.75 200 1.3 190 110 0.75 200 3.75 275 110 1.0 200 1.3 180 110 0.75 200 1.3. 0.75 195 110 0.75 200 1.3. 0.75 200 110 0.75 200 2.6. 0.75 205 110 0.75 200 3.75. 0.75 295 110 0.4 200 1.3. 0.75 1.3 195 110 0.75 260 1.3 7.75 1.3 220 120 0.5 200 1.3 205 120 0.75 200 1.3 205 120 1.0 200 1.3 0.75 240 120 0.75 200 1.3 0.75 340 110 0 20 3.75 230 110 0.75 20 3.75 280 - Pounds per ton 110 - HMW acrylamide, acrylic æ id copolymer, anionic, Mw -10 to 15 million 120 - HMW acrylamide, DM~EA co~olymer, cationic Mw~ 5 to 10 million 200 - Crosslinked ePi/DMA~ LMW cationic ~w ~50,000 260 - Linear epi/DMA, LMW cationic polymer Mw~20,000 Colloidal sllica - 4 - 5 nm 270 - Poly aluminum chloride and 260 (95:5 mole ratio) ationic Starch - Cationic potato starch, 0.035 degree of substitution Example 2 1 321 0~ 6 500 mls. paper stock mlxed wlth the following additives added while mlxlng the sample at 1000 rpm. The additives were added at 5 second lntervals.
1. Low molecular weight cationic polymer.
2. High molecular weight polymer 3. Colloidal slllca The samples were then drained through a laboratory drainage tester with the first 5 seconds of filtrate being collected for testinq. The results are provided in Table II.

Table il 13 21 0 4 6 MW Polymer LMW Polymer Colloldal Orainaae roduct drY(lb/Ton) ~ Silica(lb/Ton) mLs/5sec 0.5 0 0 155 110 0.75 200 1 2 245 110 0.75 200 2 2 325 110 0.75 200 3 ~ 340 110 0.75 200 1 0 210 110 0.75 200 2 o 265 110 0.75 200 3 o 295 110 0.75 210 1 230 1~0 0.75 210 2 310 110 0.75 210 2 305 110 0.75 210 3 340 110 0.75 210 2 2 365 110 0.75 220 1 260 110 0.75 220 2 285 110 0.75 220 3 305 110 0.75 230 1 265 110 0.75 230 2 285 110 0.75 230 3 315 110 0.75 240 1 265 110 0.75 240 2 2 295 110 0.75 240 3 295 110 0.75 250 1 140 110 0.75 250 2 150 110 0.75 250 3 180 110 0.75 260 1 195 110 0.75 260 2 230 110 0.75 260 3 235 110 0.75 270 1 170 110 0.75 270 2 220 110 0.75 270 3 250 LMW Cationic Pol~mers:
200 - Crosslinked eoi/DMA, LMW cationic Mw - 50,000 60 - Linear epi~DMA, L~W cationic Polymer Mw -20,000 210 - EOC/ammonia cooolymer Mw ~ 30,000 220 - oolyoADMAct- lOO,OOCMW
230 - PolyOAOMAC, _ 150,000MW
40 - PolyDAOMAC,~ 200,000 MW
50 - Acrylamide, OMAEM MCQ copolymer, HMW (MCQ=methyl chloride quat), Mw-10 to 15 million 70 - Poly aluminum chloride and 260 (95:5 mole ratlo) olloidal Silica - 4-5nm, dosage on dry basis 10 - Acrylic æ id, æ rylamide caoolymer, HWM anlonic, Mw- 10 to 15 ml1 n - 10 -3 2 1 ~ 4 6 66530-452 ~ le 3 Plant A has a six vat, cylinder machine currently pro-ducing recycled board for various end uses. Weights range from 50 to lS0 lb./3000 sq. ft. with calipers in the 20-40 pt. range.
The furnish is 100% recycled fiber.
The current program consists of the following:
1. LMW 200 as a coagulant is fed to the machine chest at dosages typically between 1 and 6 lb. per ton as needed to con-trol the charge in the vats between - 0.02 and 0.01 MEQ./ML
(milliequivalents per millilitre).
2. HMW 110 fed as a flocculant after the screens to each individual vat through a bank of rotometers to control dosage.
Dosages are typically in the range of 1 to 4 lb. per ton as needed for retention and drainage profile modification.
- 3. Colloidal silica fed directly into the post-dilution water for the HMW 110. After mixing with the dilution water and the HMW 110, passes through a static mixer, a distribution header and then through the rotometers mentioned above and onto the machine. Typical dosages to date have been in the range of 0.5 to 1.0 dry pounds per ton.

4. A cationic pregellatinized potato starch with .025 d.s.
is added on one very high strength grade at 40 lb. per ton for added Ply-Bond. Bags of the starch are normally thrown into the beater at 15 minute intervals (depending on production rate) by the beater engineer.
With the addition of the colloidal silica in the 0.5 to 1.0 lb. per ton (all colloidal silica dosages should be assumed to be in Dry lb. per ton unless stated otherwise) to dual polymer program we have seen the following results:
1. Within 10 minutes of adding the silica, sheet moisture dropped from 7.5% to 1.5% moisture. This in turn resulted in the - 12 - 1 3 2 1 ~ ~ 6 66530-452 backtender reducing the steam in the high pressure dryers from 120 to 70 PSI.
2. After moistures were again in line, the machine was sped up 10 to 15~ without putting all the steam back in. On some of the heavier weights we have actually run out of stock before reaching their normal steam limited condition. On the lighter weight grades we normally run out oE turbine speed before run-ning out of steam. Steam savings even on the lighter grades are significant, normally 10 to 30%.
3. Vat drainage rates increased 30 to 50%. In general the vat drainages went from an initial 35 to 40 Schoppler-Riegler Freeness to a 15 to 20 level. The same results were seen using a laboratory drainage tester which increased from 150 mL/5 sec.
to nearly 300 mL/5 sec. for a 500 mL. sample at 0.5 - 1.0~ con-sistency. The vat level controls responded by adding more dilution water which lowered the pond consistency and resulted in a much improved sheet formation.
4. Retentions improved from a typical 85 to 92% up as high as 99~ on the heavier weights. In general retention was improved significantly, to the point in fact that there were so few solids going to the saveall that we were having a very difficult time forming a mat without sweetener stock. On the lightest weight grades retention improvements of 10 to 25% were achieved over and above a reasonably well optimized dual polymer program.

5. Ply bonding, Mullen, and cockling were also improved as a result of the addition of the silica. On their heavily refined grades they generally have to slow way back due to severe cock-ling and slow drying. The addition of the silica eliminated much of this problem and they have been able to speed up to record production rates on these grades. Ply Bond and Mullen also improved 10 to 30 points primarily due to better formation.

:

~ 13 - 1 ~ 21046 66530-452 6. It is very important to note that the addition of starch is in no way necessary to the performance of this program. We have run both with and without starch and have never seen the starch have any bearing on program performance.

Claims (5)

1. A method for dewatering paper furnish in a papermaking process, which method comprises the steps of adding to the paper furnish from 0.1 to 25 pounds per ton on a dry basis, based on furnish, of a low molecular weight cationic organic polymer having a molecular weight of at least 1000 up to about 500,000; and then adding collodial silica with an average particle size within the range of from 1 to 100 nm; and 0.1 to 5 pounds per ton on a dry basis, based on furnish, of a high molecular weight charged acrylamide copolymer having a molecular weight of at least 500,000, the collodial silica and the high molecular weight charged acrylamide copolymer being added in either order.

2. The method of claim 1, wherein the high molecular weight charged acrylamide copolymer is an anionic polymer.

3. The method of claim 1, wherein the high molecular weight charged acrylamide copolymer is a cationic polymer.

4. The method of claim 1, wherein the low molecular weight cationic polymer is selected from the group consisting of diallyldimethylammonium chloride polymer, epichlorohydrin/dimethylamine copolymer, and ethylene dichloride/ammonia copolymer; and wherein the high molecular weight charged acrylamide polymer is selected from the group consisting of acrylic acid/acrylamide copolymer, dimethylamino ethylacrylate quaternary/acrylamide copolymer; dimethylamino ethylmethacylate quaternary/acrylamide copolymer.

5. The method of claim 1, wherein the low molecular weight cationic polymer and the silica are present in a weight ratio of low molecular weight cationic polymer to silica of from 100:1 to 1:1; and the high molecular weight charged acrylamide copolymer and the collodial silica are present in a weight ratio of high molecular weight charged acrylamide to silica of from 20:1 to 1:10.